The conversion of solar energy into clean fuels such as hydrogen is a promising energy source with the potential to turn solar into a portable, energy-dense form for transportation applications as well as a reliable, practical primary energy source for utility-scale generation. Disadvantageously, the limitation of the solar flux at the earth's surface necessitates large land areas for a solar photovoltaic utility to provide power on par with a traditional power plant. The difficulty of acquiring so much contiguous land area, the cost of the purchase, and the potential consequences to wild habitat are all concerns for large-scale solar utilities. These and other factors make solar photovoltaic harvesting of energy from the sun challenging to accomplish at a large scale. Efforts have been marked by a need for large land areas to produce power equivalent to a conventional fossil fuel utility. Furthermore, the intermittency of sunlight prohibits reliable baseload power generation and necessitates the use of energy storage to provide consistent electricity production. These challenges have been an impediment to solar energy becoming a reliable, continuous energy source as an alternative to fossil fuels.
Electrolysis of liquid water to obtain hydrogen gas as a fuel source is another approach that has been tried. Existing electrolysis technologies include alkaline water electrolyzers and proton exchange membrane (PEM) electrolyzers. Both approaches rely on liquid water electrolyte to achieve sufficiently high current densities (about 1 ampere/cm2, which is shown elsewhere as A/cm2) at commercial application levels, which would require large quantities of freshwater for a utility-scale solar fuel production system. Tailoring proton exchange membrane electrolyzers for use with ambient humidity, including seawater vapor, and improving the performance and efficiency of these systems are the subject of present embodiments.
Besides the large quantities of freshwater that would be needed, water that is input to these electrolyzers might be further purified to deionized (DI) water to prevent the impurities from depositing on the catalysts and increasing the overpotential of the water-splitting reaction (i.e, electrolysis). However, to do so with freshwater, at the type of scale that would be desirable, raises its own difficulties in terms of limited resources in a water-scarce environment. Further, there is the challenge of having to pump liquid water that results in an energy loss in these systems. Potentially, the abundant water resources of the ocean could serve as the feedstock for hydrogen generation, but electrolysis in seawater is hampered by the salts and impurities of the oceans. The impurities in the seawater, which cause fouling of the catalyst marked by electrochemical reduction of impurity metals onto the cathode of the electrode array, tend to decrease the current density that might otherwise be achievable and leading to irreversible degradation of the voltage-current density association.
Accordingly, there is a defined and tangible benefit associated with achieving efficient and stable solar hydrogen production through electrolysis, as a replacement to hydrogen generation from fossil fuels, and which can be accomplished in ways other than through the use of liquid freshwater as a feedstock. Success in this regard would be characterized by avoiding the steps and costs associated with active water purification and the reliance upon deionized water to mitigate impurities.